The head of the newly-found Josephoartigasia monesi (A), in comparison to a South American rodent known as a pakarana, Dinomys branickii (B).

Image: G. Lecuona [larger size].

As a resident of NYC, you often hear stories of enormous rats that are aggressive enough to fight an alleycat — and win. Even though I watch the rats run around in the subways and am impressed by their audacity, I’ve never seen any rats that had reached a particularly impressive size, although I have seen some rather large specimens. So I am probably not looking for these monster-rats in the right places. In fact, if I wish to see an astonishingly large rodent, I should be looking for them in South America, which is filled with them. In fact, scientists just discovered the fossil remains from a truly huge rodent, Josephoartigasia monesi, which weighs as much as a mid-sized car and is approximately the same size as one, too.
However, if you are thinking that this giant rodent looks like an outsized rat, well you’d be wrong. According to an artist’s rendering, which is based on the very well-preserved fossil skull, the animal actually looks more like a guinea pig (see image, above).

The fossil skull is 21 inches (53 centimeters) long (figure 1, below). Based on its large size, researchers estimate that J. monesi weighed approximately 2,200 pounds (1,000 kilograms), and was approximately ten feet long. It is thought to have lived between 2-4 million years ago.

“It’s a beautiful piece of nature,” said paleontologist and study co-athor Ernesto Blanco of the new fossil species. “You feel the power of a very big animal behind this.”

The fossil skull was found in a cliff face overlooking the River Plate estuary near the South Atlantic Ocean in Uruguay. Argentinian amateur fossil collector, Sergio Viera, discovered the fossil skull in 1987 and donated it to the National History and Anthropology Museum in Montevideo, Uruguay. Unfortunately, it remained packed away inside a dusty crate in the museum collections until recently. Curator Andrés Rinderknecht, rediscovered it and with the help of Blanco, wrote a formal description of this extinct species, which is new to science.

The animal had extremely broad incisors, suggesting that it could bite — very hard. These large teeth might have come in handy if the animal was fighting for mates or protecting itself from predators. In contrast, its molars were quite small so it probably subsisted on a soft vegetarian diet.

“It probably fed on aquatic plants and fruits, because its molars are small and not good for grass or other abrasive [vegetation],” Rinderknecht observed.

According to the paper, J. monesi inhabited forests around river deltas or estuaries.

“From what we can tell, we know it was a herbivore that lived on the shores of rivers or alongside streams in woodland areas,” Rinderknecht said. “Possibly it had a behavior similar to other water-faring rodents that exist today, such as beavers, which split their time between land and water.”

But this newest discovery is not the only large rodent species known. Previously, the largest rodent known to have ever existed was Phoberomys insolita, which is estimated to weigh more than 1,500 pounds, based on a very few fossil remains. However, a slightly smaller congener, Phoberomys pattersoni, is known from a nearly complete skeleton that was discovered near Urumaco in Venezuela in 2000. That rodent is thought to have been approximately the size of an American bison and to have weighed up to 1500 pounds (680 kilograms). Both of these animals lived approximately 8 million years ago.

Why is there such a tremendous diversity of fossil rodent of all sizes in South America? Because South America was an island for so long, rodents evolved in isolation to exploit a wide variety of niches that might have otherwise been closed to them if large hoofed mammals were present. Secondarily, large body size can allow animals to utilize low-quality food resources, such as wood, that smaller species cannot digest. Further, once large body size evolved, it also acts as a deterrent to the many predators that were no doubt present at the same time.

However, when North and South America were joined by the Panama land bridge around 3 million years ago, this allowed plants and animals — including large predators and herbivores — to move from one continent to the other, which might have led to the extinction of these large rodent species.

The authors are already planning to locate more of the rodent’s skeleton.

Even though I watch the rats run around in the subways and am impressed by their audacity, I’ve never seen any rats that had reached a particularly impressive size, although I have seen some rather large specimens. So I am probably not looking for these monster-rats in the right places. In fact, if I wish to see an astonishingly large rodent, I should be looking for them in South America, which is filled with them.

I’ve seen the same kinds of subway rats here in Boston. I’ll avoid those big things, thanks.

OMG, it’s a real-life ROUS! I love all animals, but I make an exception for rodents, so this story had an element of the grotesque for me, as well as being a really neat find.

Posted by: Library Diva | January 18, 2008 12:03 AM

I love animals too, but I start becoming ambivalent when they are big enough to look at me as a hot meal! LOL! I love gold fish, but when I had 12 foot hammerhead sharks under my boat in the Florida Keys, all the warm fuzzies vanished instantly! So I understand your exception for the rodent! LOL!
Dave Briggs :~)